The present invention relates to optimization of antenna sum and difference patterns, and in particular, to a sidelobe suppression arrangement for a monopulse antenna using sum and difference patterns to track targets.
As is generally known, a monopulse antenna may be subdivided into sections, for example, by using horns or quadrants, and the radar then senses the target displacement by comparing the amplitude and phase of the echo signal for each horn or quadrant.
The RF circuitry for a conventional antenna divided into quadrants subtracts the output of the left pair from the output of the right pair to sense any imbalance in the azimuth direction (azimuth difference pattern) and the output of the top pair from the output of the bottom pair to sense any imbalance in the elevation direction (elevation difference pattern). See Radar Handbook, Merrill Skolnick, McGraw Hill, 1970. The subtracter outputs, i.e., the difference patterns, are zero when the target is on axis, increasing in amplitude with increasing displacement of the target from the antenna axis.
A sum signal, usually representative of the energy received over the entire aperture, is generated and used as a reference signal, for video input, and for gain control.
There are many trade-offs in feed design and radiation patterns because optimum sum and difference signals, low sidelobe levels, polarization diversity, compactness, and simplicity cannot all be fully satisfied simultaneously, especially when using a single feed. Historically, a common approach has been to optimize the sum pattern and to tolerate the resulting difference pattern signal. However, it is generally regarded that optimizing undesirable features of the difference patterns are important in eliminating significant tracking problems. See Corlin, U.S. Pat. No. 4,525,716; June 15, 1985. For example, high sidelobes in the difference signals increase radar susceptibility to interference from background clutter or other off axis sources of radiation which results in tracking error and loss of efficiency.